Hydraulic cement that has set and completely hardened, as well as hardened or fired pottery or ceramic matrices, are porous, brittle and relatively weak. They crack easily and are difficult to permanently repair. In fact, replacement is generally the best and most frequent alternative.
Two different techniques have in the past been attempted to improve their physical properties. The first is to modify the wet paste prior to hardening or firing by incorporating liquid "modifiers", particulate admixtures or fibrous reinforcement. The second alternative uses chemicals (generally liquids) to surface-coat or to migrate into the hardened matrix material. Similarly, after crack development or breaking, adhesives and fillers which may or may not slightly penetrate into the matrix may be employed for repair, to form bridges between the edges of cracks or to inhibit further crack propagation. Another method, where there is surface spalling, pothole development or the formation of large cracks, fills the damaged areas (either with or without the removal of loose debris) with specialized concrete, cements or mortars. These solutions have not been particularly successful in creating or repairing cementitious materials in ways that either restore them or leave them suitable for high-performance applications where tensile strength, resistance to new or further cracking or breakage, flexibility or ductility are desirable properties.
For purposes of this application, the word "cement", used alone or in combination with other words, refers to any type of cementitious material made with water and one or more inorganic ingredients to form a paste which, through reactive processes, creates hydrogels to bind it together. The definition includes, but is not limited to, ceramics, pottery, clay, mud, earthenware, portland, natural and fly-ash pozzolan, lime, gypsum, cementitious materials and the like.
Problems are also encountered with the sand and/or coarse aggregates (typically of an average size between 3/8" and 31/2") in cement matrices to form concrete or mortar. Many sands and coarse aggregates exhibit properties and behaviors similar to those of hardened cement, pottery or unfired or fired ceramic matrices. In addition, certain coarse concrete aggregates, particularly zeolites and dolomites, may react with the hydraulic cement that binds them together in concrete and mortar. Aggregates having high levels of porosity and low strength also cause problems. Thus, many commonly occurring aggregates are banned from use in concrete because of such problems.
Chemistry, morphology, geometry and micro-morphology of hardened man-made matrices and natural aggregate determine how aggressively decomposition reactions proceed, often as a consequence of alkali/silica reactions (ASR). To ameliorate some of these problems, aggregates have been soaked in water, salts, acids or even strong alkalis, but the results have been inconsistent and often disappointing. Thus, when a local aggregate is unacceptable for use in concrete, suitable aggregate must either be imported or man-made, often at considerable expense.
The present invention seeks to alleviate problems of this kind by treating natural or man-made cement and/or aggregates with supercritical fluids to alter their surface, surface layer, and in some cases their entire body chemistry and/or microstructure.
As is well known from the literature, every pure substance has a certain, temperature/pressure threshold, above which, when simultaneously exceeded, all the properties of the substance continuously change without exhibiting any gas/liquid phase discontinuity. These are known as the "critical pressure" and "critical temperature" (T.sub.c and P.sub.c) of a pure substance. The combined critical pressure and critical temperature threshold is called the "critical point" of a pure substance. A substance or mixture that has been elevated above the critical point is "supercritical". The term "supercritical fluid" refers to substances, mixtures and solutions which are in this state. Supercritical pure substances, mixtures and solutions exhibit varying degrees of solvent behavior and reactivity with other elements and compounds.
As is disclosed in U.S. Pat. No. 5,518,540, issued May 21, 1996 (the "'540 patent"), the disclosure of which is incorporated herein by reference, when a hardened, porous cement or ceramic matrix is exposed to supercritical carbon dioxide ("scCO.sub.2 "), alone, the scCO.sub.2 infuses into the matrix and causes chemical and microstructural changes that can produce desirable properties and behavior in the matrix.